Hello,
1) Anyone who needs to call you must dial your US number as if you were in the US (this is how international roaming works – they call your complete US number, the call goes through the US and then Verizon redirects it back to wherever you are). So, your American friends in Europe and anyone in any of the European countries you’ll be in should dial the following: 00 1 ??? ??? ????. The last 10 digits are the area code and local number.
2) To call German telephone numbers from Germany just dial what locals dial. For example, German cell phone numbers have a national dialing format (they all start with 01 when called from Germany and do not use geographical area codes). Here’s one example: 015 ???? ????. This is what you will dial. For more info please refer to the link below:
https://www.howtocallabroad.com/results.php?callfrom=germany&callto=germany
3) To call a German cell phone from any other European country you should leave out the starting zero above and dial: 00 49 1 ???? ????. 49 is Germany’s country code and 00 is the code you use to dial internationally from any EU country. Note that some German cell numbers may have more digits.
4) People in the US do not need to change anything in the way they dial your cell phone.